1. Field of the Invention
This invention relates to an electrode for applying an electric voltage to an optical channel waveguide of an optical waveguide element in which the optical channel waveguide is formed by proton exchange, and a method of forming the electrode.
2. Description of the Related Art
There have been provided various optical waveguide elements having an optical channel waveguide formed on a substrate. As a method of forming the optical channel waveguide, there has been known a proton exchange process.
In the proton exchange process, metal film is first formed on a surface of a substrate, an opening is formed in the metal film by etching and proton exchange is carried out on the surface of the substrate using the metal film as a mask.
Generally an electric voltage is applied to the optical channel waveguide through electrodes disposed near or just above the optical channel waveguide.
A conventional method of forming the electrodes for applying an electric voltage to the optical channel waveguide will be described with reference to FIGS. 9A to 9H, hereinbelow.
Metal film 2 such as of Cr is first formed on a substrate 1 as shown in FIG. 9A.
A resist layer 3 is formed on the metal film 2 in a predetermined pattern by photolithography as shown in FIG. 9B.
Then the metal film 2 is etched to form openings 4 in a predetermined pattern in the metal film 2 using the resist layer 3 as a mask, and the resist layer 3 is removed as shown in FIG. 9C.
Thereafter proton exchange is carried out using the metal film 2 with the openings 4 as a mask, thereby forming optical channel waveguides 5 on the surface of the substrate 1 as show in FIG. 9D.
The metal film 2 is then removed by etching as shown in FIG. 9E and the substrate 1 is annealed as required.
Thereafter a conductive film 7 such as of aluminum is formed over the surface of the substrate 1 as shown in FIG. 9F.
A resist layer 8 is formed over the conductive film 7 with portions opposed to the optical channel waveguides 5 exposed by photolithography as shown in FIG. 9G.
Then the conductive film 7 is removed at the portions opposed to the optical channel waveguides 5 by etching using the resist 8 as a mask as shown in FIG. 9H.
When the resist 8 is thereafter removed, the conductive films 7 are left on opposite sides of each optical channel waveguide 5. The conductive films 7 on opposite sides of each optical channel waveguide 5 can be used as electrodes for applying an electric voltage to the optical channel waveguide 5.
However this method is disadvantageous in the following point. That is, when the resist mask 8 is formed over the conductive film 7 with the portions opposed to the optical channel waveguides 5 exposed, the edge of the resist mask 8 circumscribing the optical channel waveguide 5 cannot be precisely aligned with the edge of the optical channel waveguide 5 due to fluctuation in skill of the operator and/or in precision of the exposure device. Accordingly, the edges of the electrodes (conductive film) hang over the optical channel waveguide or are positioned away from the edge of the optical channel waveguide 5 as shown in FIG. 10 in an enlarged scale, which results in fluctuation in performance of the optical waveguide element or deterioration in yield. In FIG. 10, L denotes the alignment error.
In Japanese Unexamined Patent Publication No. 7(1995)-146457, there is disclosed a method of forming the electrodes for a optical waveguide element which can overcome such a problem. In the method, the metal film which is used as a mask for setting the pattern of the optical waveguide upon proton exchange is left there and used as the electrodes. That is, metal film is formed on a surface of a substrate, openings of predetermined shapes are formed in the metal film, proton exchange is carried out on the surface of the substrate with the metal film used as a mask, thereby forming optical channel waveguides, and the metal film is removed with at least a part of the edges of the openings left there. The metal film fractions are used as the electrodes.
In the method the openings of predetermined shapes are formed in the metal film generally by etching though liftoff may be used.
When metal film is processed by etching or liftoff, the thickness of the metal film should be several hundred namometers (nm) at most. When such thin metal film is used as an electrode, the resistance of the electrodes becomes high and accordingly optical waveguide elements provided with such electrodes are hard to operate at high speed (e.g., high speed modulation at several hundred MHz or higher).
In view of the foregoing observations and description, the primary object of the present invention is to provide an electrode for an optical waveguide element which is formed with its one edge precisely aligned with one edge of the optical channel waveguide and at the same time makes it feasible to operate the optical waveguide element at high speed.
Another object of the present invention is to provide a method of forming such electrodes for an optical waveguide element.
In the method of the present invention, a part of the metal film which is used as a mask for setting the pattern of the optical waveguide upon proton exchange is left there as in the above identified Japanese patent publication (Japanese Unexamined Patent Publication No. 7(1995)-146457), then the metal film is plated and used as the electrodes.
That is, in accordance with a first aspect of the present invention, there is provided a method of forming electrodes for an optical waveguide element comprising the steps of
forming a metal film on a surface of a substrate,
forming openings of predetermined shapes in the metal film,
carrying out proton exchange on the surface of the substrate with the metal film used as a mask, thereby forming optical channel waveguides,
leaving at least a part of edge portions of the metal film defining the openings,
plating the metal film with plating metal, and
processing the metal film plated with the plating metal into electrodes of predetermined shapes for applying an electric voltage to the optical channel waveguides.
In the method of forming electrodes for an optical waveguide element in accordance with a second aspect of the present invention, the metal film used as a mask in the proton exchange is first processed into metal film fractions of predetermined shapes corresponding to the shapes of electrodes to be formed, each metal film fraction including at least a part of an edge portion defining one of the openings, and then the metal film fractions are plated with plating metal and used as the electrodes for applying an electric voltage to the optical channel waveguides.
In the method of forming electrodes for an optical waveguide element in accordance with a third aspect of the present invention, in the method of the first or second aspect of the present invention, negative photo-resist is applied to the substrate after said proton exchange and before said plating, then the photo-resist is exposed to light from the back side of the substrate using the metal film as a photo-mask, the photo-resist is subsequently removed with the part of the photo-resist which is on the optical channel waveguides and accordingly exposed to light left there, and then the plating is effected using as a mask the part of the photo-resist left on the substrate.
In accordance with a fourth aspect of the present invention, there is provided an electrode for an optical waveguide element which is formed on a substrate, on which an optical channel waveguide is formed by proton exchange, with its one edge aligned with one edge of the optical channel waveguide and is for applying an electric voltage to the optical channel waveguide, wherein the improvement comprises that
the electrode comprises a metal film fraction which is a part of metal film used as a mask when the optical channel waveguide is formed by the proton exchange and a plating metal layer formed on the metal film fraction by plating.
Preferably a buffer layer is formed between the substrate and the metal film.
The metal film used as a mask for setting the pattern of the optical channel waveguide upon proton exchange naturally has an edge aligned with an edge of the optical channel waveguide. Accordingly when an electrode is formed by plating the metal film including at least a part of an edge portion defining one of the openings, the edge of the electrode can be precisely aligned with the edge of the optical channel waveguide.
When the metal film plated with metal is used as an electrode, the thickness of the electrode increases and the resistance of the electrode lowers as compared with when the metal film is used as an electrode as it is. Accordingly the optical waveguide element in which an electric voltage is applied to the optical channel waveguide through the electrode can be operated at high speed.
When the plating metal layer comes to hang out over the optical channel waveguide from the edge of the metal film, the finished electrode cannot have an edge aligned with the edge of the optical channel waveguide even if the metal film on which the plating metal is plated has an edge aligned with the edge of the optical channel waveguide, which results in the same problem as that described above in conjunction with FIG. 10.
The method in accordance with the third aspect of the present invention can overcome this problem. That is, when the negative photo-resist applied to the substrate after the proton exchange is exposed to light from the back side of the substrate using the metal film as a mask, the exposed part of the photo-resist has an edge precisely aligned with the edge of the optical channel waveguide. Accordingly, by removing the negative photo-resist with the exposed part left there and effecting the plating using the exposed part of the photo-resist as a mask, the plating metal cannot hang out over the optical channel waveguide and the edge of the electrode plated with the plating metal can be precisely aligned with the edge of the optical channel waveguide.
When a buffer layer is formed between the substrate and the metal film, light propagation loss due to the electrode can be reduced.